Waste paper, such as old newspapers and magazines, is an important raw material for the pulp and paper industry. However, the reutilization of waste paper requires special treatment to permit reincorporation into various kinds of paper.
Firstly, the waste paper must be disintegrated in water to obtain a suspension of separated fibres. Two general types of equipment for this purpose are well known in the pulp and paper industry: low-consistency pulpers operating below 5% consistency (i.e. dry substance concentration) and high-consistency pulpers operating above 8% consistency, typically at 10-20%. Due to the presence of dispersed printing ink the pulp will be grey, and it is mostly used for production of grey cardboard.
Another type of treatment consists in disintegrating printed waste paper in the presence of chemicals in order to remove the ink. This so-called deinking combines pulping with chemical action to release the ink particles from the fibres. The treatment is carried out in a pulper at alkaline conditions, the most common deinking chemicals being sodium hydroxide, sodium silicate, and hydrogen peroxide; a surfactant is also added to facilitate the removal of ink. Deinking is generally made at 30.degree.-50.degree. C. The pH will generally be above 9, and in case of deinking at high consistency (above 8%) a pH above 10 is usually required for satisfactory deinking. The high pH leads to some yellowing of the pulp; this is reduced by the addition of hydrogen peroxide.
After this ink-releasing step, the pulp is diluted and purified to remove various contaminants, and the ink particles are removed from the pulp by washing, flotation or a combination of these.
It is known to use alkaline cellulase to improve the deinking during low-consistency pulping. Thus, JP-A 59-9299 (Kao) describes the use of alkaline Bacillus cellulase at 3-6% consistency and 40.degree.-45.degree. C. The process comprises 20 minutes pulping followed by 60-90 minutes maceration. Deinking chemicals were added at the beginning of the pulping, and cellulase and surfactant were added at the beginning of pulping or at the beginning of maceration.
JP-A 63-59494 (Honshu Paper) describes the use of alkaline cellulase from Humicola insolens to improve deinking at 5% consistency and 50.degree. C. The process comprises 3-5 minutes pulping followed by maceration for 140-360 minutes. Deinking chemicals, surfactant and cellulase were added before or during the maceration. The pH was 9.1.
EP 262,040 (Cellulose du Pin) and EP 351,655 (Cultor) describe the use of acidic cellulase and hemicellulase during pulping at 2-5% consistency and pH 3-7 for a different purpose (improvement of drainage). No deinking was performed.
It is described in TAPPI Journal, June 1989 that the effect of cellulase on pulp has a maximum at 3% consistency and decreases from 3 to 5%.
Use of cellulase during high-consistency pulping (above 8%) has not been described. It is the object of the invention to provide an improved process for deinking of waste paper using alkaline cellulase.